1. Field of the Invention
The present invention involves controlling the deposition attributes such as deposition rate and composition of an epitaxial layer being fabricated from a plurality of atomic species in a vacuum chamber.
Many widely known deposition techniques such as molecular beam epitaxy (MBE) and electron-beam evaporation are used in the fabrication of many types of devices including electronic and opto-electronic devices. An important need exists to improve both the accuracy and the yield of high performance electronic and opto-electronic device fabrication. Some examples of these devices are Resonant Tunneling Diodes (RTD), Vertical Cavity Surface Emitting Lasers (VCSEL), electro-optical modulators, and quantum well lasers. Often, the process for making these devices involves depositing thousands of distinct epitaxial layers while maintaining strict control of the composition and thickness of each layer. In addition, there are further constraints such as:
1) limited access of the monitoring system to the vacuum deposition chamber.
2) monitoring system has to be non-invasive to the deposition process.
3) error due to noise and drift has to be sufficiently small during the course of the deposition.
4) monitoring system must have a high rejection of stray light from the surrounding environment.
5) monitoring system must not be affected by special requirements such as sample rotation during MBE deposition.
6) the monitoring system should be portable and most of the setup should be remote from the deposition chamber.
Currently, many epitaxial deposition techniques such as electron-beam evaporation and Molecular Beam Epitaxy (MBE) lack good deposition monitoring systems which have the ability to monitor in real-time the deposition rate as well as composition during multi-component epitaxial deposition. Included among these monitoring systems is the technique of atomic absorption.
It has long been recognized that atomic absorption can be used as a tool for monitoring of material deposition processes. Atomic absorption techniques generally involve passing a light beam through the molecular beam of the MBE process and then measuring the resulting intensity of the light beam. The more atoms that are being deposited, the greater the atomic absorption of the light, resulting in a lower intensity of the light remaining. The light beam must have a wavelength that corresponds with the atomic species desired to be measured. In this way the deposition of the epitaxial layers with respect to a particular atomic species can be measured. Recently, because of a need for better non-invasive real-time feedback during the deposition process, increased attention has been turned toward monitoring techniques using atomic absorption of the molecular beam flux.
2. Description of the Related Art
There have been many prior monitoring systems which utilize the atomic absorption technique. However, the prior art discloses the monitoring of just one channel at a time with the possibility of combining several independent one-channel units to build a multi-channel system. These multi-channel systems do not integrate the channels and therefore require additional port space in the vacuum chamber.
Appl. Phys. Lett. 60 (5) Feb. 3, 1992, p. 657, (Klausmeier et al.), discloses the passing of a modulated light beam through the molecular beam of an MBE process and then passing the light beam through a bandpass filter and into a photomultiplier tube. The filter passes only a particular emission line so that only the deposition of the atomic beam flux corresponding to that particular wavelength is measured. If more than one atomic beam flux is being deposited on the substrate then the Klausmeier device can only measure the deposition rate and composition of one atomic species.
J. Vac. Sci. Technol. B 12(2), Mar/Apr 1994, p. 217, and J. Vac. Sci. Technol. A 13(3), May/Jun 1995, p. 1797, disclose a system similar to Klausmeier's and the implementation of multiple channels is not directly addressed.
Appl. Phys. Lett. 63 (23), Dec. 6, 1993, p. 3131, and Appl. Phys. Lett. 65 (1), 4 July 1994, p. 4, disclose a dual-beam configuration with two channels but it is not optimized for the limited optical access that exists in most MBE and electron beam evaporation systems. The two channels are not integrated to follow one path through the molecular beam and therefore require additional port space in the vacuum chamber.